The problem addressed by the present invention is how to create an optical position sensor that has high precision, that can be read remotely, that has self-fault detection, and that can be multiplexed with other sensors. One previous solution to this problem, exemplified by U.S. Pat. No. 4,849,624, utilizes wavelength division multiplexing to transmit an optical interrogation signal to the sensor. At the sensor, the interrogation signal is demultiplexed, and one wavelength component is allowed to strike each encoder track. After being reflected or transmitted by the tracks, the different wavelength components are combined onto an optical fiber and transmitted back to a detector. At the detector, the return signal is demultiplexed, to thereby read the encoder pattern and determine the position of the encoder.
Time-division multiplexing techniques have also been used for reading encoders. For example, in the system described in U.S. Pat. No. 4,641,025, an optical interrogation pulse is divided into N channel pulses, one for each track, and each channel pulse is subject to a different time delay. The presence or absence of a return pulse in a given time slot then indicates the state (reflecting/nonreflecting) of an individual track.
A third prior approach to optical position sensors has been to employ analog rather than digital techniques. In an analog sensor, an interrogation signal containing two wavelength components is demultiplexed at the sensor. One wavelength component is transmitted through or reflected from an analog encoder track having a continuously variable reflectivity or transmissivity. The second wavelength component bypasses the track. The two components are then combined and returned to the detector. The detector determines the ratio of the intensity of the two wavelength components, to therefore determine the encoder position.
All of the above techniques suffer from a number of limitations. Both of the digital sensors described above have the disadvantages that their fault detection is incomplete, and that multiplexing of many channels is difficult. Another disadvantage of the time-division multiplexing technique is that it requires considerable quantities of optical fibers to create the requisite time delays. For example, for a 10-bit sensor, at least 5 meters of optical fiber would need to be coiled up inside each sensor. Analog sensors have the advantage of allowing self-fault detection by monitoring both reflection and transmission. However, they rely on wavelength-multiplexed reference beams that can be affected by wavelength-dependent losses. In addition, high-resolution analog masks for such sensors are difficult to design.